Effect of light wavelength on underwater in-situ darkfield microscopy at different water optical attenuation coefficients

Underwater in-situ darkfield microscopy can achieve high-resolution high-contrast plankton images for plankton species identification, size measurement and abundance estimation. However, light is scattered and absorbed by water, and this will blur plankton images and reduce signal-to-noise ratio, especially for turbid water conditions. Different wavelength light has different optical scatter and absorption characteristics. Therefore, we investigate effect of light wavelength on underwater in-situ darkfield microscopy in different water optical attenuation coefficients. An underwater darkfield microscopy system with the spatial resolution of 10 μm has been established. The illuminators at blue, green, red and white bands are designed to generate sheet light, and then underwater darkfield images were grasped from 0.7/m to 7.04/m optical attenuation coefficients. The target-to-background contrast and intensity attenuation characteristics show that the red light has the better contrast and weaker attenuation performance than the green and blue light in turbid water environments.